This invention relates to the field of ion optics and mass spectrometry and, more particularly, radio frequency (RF) devices and methods for ion transfer, storage and preparation of ion packets for mass analysis.
Mass spectrometry employs a variety of radio frequency (RF) devices for ion manipulation. The first distinct group comprises RF mass analyzers.
Radio frequency (RF) quadrupole ion filters and Paul ion trap mass spectrometers (ITMS) have been well known since the 1960's. Both mass analyzers are suggested in U.S. Pat. No. 2,939,952. A detailed description of one example can be found in P. H. Dawson and N. R. Whetten, in: Advances in electronics and electron physics, V. 27, Academic Press. NY, 1969, pp. 59-185. More recently linear ion traps emerged with radial (see U.S. Pat. No. 5,420,425) and an axial (see U.S. Pat. No. 6,177,668) ion ejection. All ion trap mass spectrometers employ nearly ideal quadratic potential (achieved with hyperbolic surfaces) and are filled with helium at an intermediate gas pressure. Ions are trapped by an RF field, dampened in gas collisions and are sequentially ejected, e.g. while ramping amplitude of the RF field. Ion traps employ many elaborate strategies to perform ion isolation and fragmentation which (in combination with resonant ejection) allow a so-called tandem mass spectrometer (MS-MS) analysis.
In the late 1990's there appeared a trend of miniaturizing 3-D ion trap and quadrupole mass spectrometers to form parallel batches by methods of micromachining (see U.S. Pat. No. 6,870,158; Badman et. al., A Parallel Miniature Cylindrical Ion Trap Array, Anal. Chem. V. 72 (2000) 3291; and Taylor et. al, Silicon Based Quadrupole Mass Spectrometry using micromechanical systems, J. Vac. Sci. Technology, B, V19, #2 (2001) p. 557).
The second distinct group of mass spectrometric RF devices comprises ion guides. Mostly those devices are based on 2-D quadrupole or multipole, extended along one dimension and usually referred as linear. Linear ion guides are mostly used for ion transfer from gaseous ion sources to mass spectrometers like quadrupole. Gaseous collisions relax ion kinetic energy and allow spatial confining of ions to the guide (see U.S. Pat. No. 4,963,736). Gaseous linear multipoles are also employed for ion confining in fragmentation cells of tandem MS, like triple quadrupoles and Q-TOF (see U.S. Pat. No. 6,093,929). An axial DC field formed, for example, by external auxiliary electrodes, is used to accelerate ion transfer within a guide (see U.S. Pat. No. 5,847,386) or within a fragmentation cell (see U.S. Pat. No. 6,111,250).
Linear ion guides could be plugged by axial DC fields to form a linear ion trap. Multipole linear ion traps are widely used for ion accumulation and pulsed ion injection into a 3-D ITMS (see U.S. Pat. No. 5,179,278), a FT ICR (see S. Senko et. al., JASMS, v. 8 (1997) pp. 970-976), an orbitrap (see WO02078046 A2 by Thermo) and a into time-of-flight mass spectrometer (TOF MS), directly (see U.S. Pat. No. 5,763,878 by Franzen) or via an orthogonal accelerator (see U.S. Pat. No. 6,020,586 by Dresch et al.; U.S. Pat. No. 6,507,019 by Sciex; and Great Britain patent GB2388248 by Micromass). Ion guides and ion traps are also employed for exposing ions to ion molecular reactions with neutrals (see U.S. Pat. No. 6,140,638 and U.S. Pat. No. 6,011,259 by Analytica), with electrons (see British patents GB2372877, GB2403845 and GB2403590), ions of opposite polarity (see S. A. McLuckey, G. E. Reid, and J. M. Wells, Ion Parking during Ion/Ion Reactions in Electrodynamic Ion Traps, Anal. Chem. v. 74 (2002) 336-346, and U.S. Pat. No. 6,627,875 by Afeyan et al.) and photons (see Dehmelt H. G., Radio frequency Spectroscopy of Stored Ions, Adv. Mol. Phys. V. 3 (1967) 53).
A majority of mass spectrometric ion guides and linear storing ion traps devices employ a topology of quadrupole and multipole RF fields. Referring to FIGS. 1A-1D, such multipoles are composed of rods with alternated RF phase. A quadrupole ion guide (FIG. 1A) is formed by two pairs of parallel rods with an RF voltage being applied between the sets. To make a distinction, one phase is denoted as +RF, while an opposite phase of RF signal is denoted −RF. Similarly, an octupole (FIG. 1B) and a higher order multipole (FIG. 1C) are formed of two interleaved sets of rods. Multipole rods are aligned on a cylindrical surface. To eliminate a net field on axis (denoted as RF=0) usually those sets are fed by two equal RF signals of the opposite phase. In the extreme case of very high order multipole the curvature of the inscribed circle becomes negligible and a portion of such multipole looks more like a plane formed by rods with alternating RF signals (FIG. 1D).
Looking at multipoles in a more general sense, one can treat the rod structure as a set of dipoles (FIG. 1D), each formed by pairs of neighbor rods. In the case of multipoles, those RF dipoles are aligned within a circular surface. Each dipole has a very short penetration range, much shorter compared to individual rods. Even at moderate spacing between dipoles their fields become independent and allow a flexible arranging of dipoles.
Referring to FIGS. 2A-2D, enclosed RF surfaces have been used for ion trapping and ion guidance. In E. Teloy and D. Gerlich, “Integral Cross Sections for Ion Molecular Reactions. 1 The Guided Beam Technique”, Chemical Physics, V. 4 (1974) 417-427, an ion source is formed using horse shoe electrodes with alternating RF signals (FIG. 2A). RF dipoles repel ions from the walls. The top and the bottom sides are plugged by DC caps. The central core of the source is almost field-free, which is convenient for ionizing by electrons and for ion relaxation in gas collisions. Referring to FIG. 2B, a so-called RF channel is formed between two planes of linear RF dipoles formed of parallel wires with alternated RF signals (see European Patent No. EP1267387 by Park). DC plugs are used on the sides of the channel.
A ring ion guide (see FIG. 2C) (see Gerlich D. and Kaefer G., Ap. J. v. 347, (1989) 849 and U.S. Pat. No. 5,572,035 to Franzen) is another example of an enclosed RF surface with a short range ion repulsion near the walls and a field-free core. For ion propulsion, a moving wave is formed by applying several RF signals with a distributed phase shift (see U.S. Pat. No. 5,818,055 and U.S. Pat. No. 6,693,276 by Weiss et al.), or a wave of DC signals is superimposed on the top of alternating RF signals (see European Patent No. EP1271608 and EP1271611 by Micromass in 2002].
Operation of various ion guides is based on the ion repelling action by inhomogeneous RF fields. The effect has been analyzed by LD. Landau and E M. Lifshitz in Theoretical Physics, Vol. 1, Pergamon, Oxford, (1960) p. 93, as well as by H. G. Dehmelt in “Advances in Atomic and Molecular Physics”, ed. D. R. Bates, Vol. 3, Academic Press, New York, (1967) pp. 53-72. Ion motion is composed of fast oscillations within an RF field and a slow motion in a mean, time-averaged force of an RF field. When there is sufficient frequency, the ion oscillations become minor compared to the geometric scale of the RF field homogeneity. The mean effect of such RF oscillations being averaged over the cycle of the RF field is equivalent to a net force that is directed towards a region with smaller amplitude of RF field. Such force is considered as a gradient of so-called dynamic potential. A slow (average) ion motion can be then approximated by ion motion within a total (effective) potential V* being a sum of dynamic D and electrostatic potentials Φ:V*(r)=D(r)+Φ(r)=zeE(r)2/4mω2+Φ(r)  (1)
Where ze and m are the charge and mass of ions, ω is the circular frequency of the RF field, and E(r) is the strength of the local RF field. The first term of the equation ties dynamic potential D to a local strength of the RF field E: D˜E2, i.e. D increases near sharp edges and zeroes on axis of symmetric RF devices. In other words, the RF field repels ions from areas with strong RF field into areas with a smaller field, usually occurring on the axis of symmetric devices.
The above cited paper (Teloy et. al, 1974) describes a generic recipe of forming ion guides and traps: “ . . . which show absolute minima of V* (total effective potential in Equation 1) in two or three dimensions of space and therefore are able to guide or to trap ions. For instance, ion traps can be constructed, in which a nearly field-free volume is enclosed by steep repulsive walls of the effective potential. Such a wall can be formed by an arrangement of equally spaced parallel rods, which are concerned alternately to RF voltages of opposite phase, or similarly by metal plates or wires.”
U.S. Pat. No. 5,572,035 to Franzen recognizes that an RF dipole surface can serve as an independent construction unit (see FIGS. 3A-3D) for repelling ions of both polarities. Particular RF surfaces are formed of two interleaved planar arrays of electrodes (see FIGS. 3B and 3C), such as wire tips in both arrays or a honeycomb mesh in combination with an array of penetrating tips (see FIG. 3A). Such surfaces are composed of RF dipoles and they are characterized by strong, but very short-ranged, ion repulsion. Franzen suggests guiding ions above the dipolar RF surface or between two dipolar RF surfaces. There is also suggested an ion guide with a different topology RF surface formed by a pair of interleaved helixes (see FIG. 3D).
U.S. Pat. No. 6,872,941 to Whitehouse et. al. suggests ion confining between an RF dipolar surface and a DC field for guiding ions, trapping ions and for pulsing ions into a TOF MS. Whitehouse et al. allows forming a narrow ribbon of ions, reducing phase space of the beam and accommodating a large number of ions without space charge effects. To eject ions into a TOF MS, the RF signals are switched to voltage pulses (see FIG. 4A). Alternatively, ions are thrown onto an RF surface for surface-induced dissociation prior to injection into TOF MS.
WO2004021385 suggests using a planar RF dipolar surface for ion manipulation between individual open traps near the surface. Ions are trapped by applying an attracting DC voltage and a short range repelling RF voltage to a spot or a thin line electrode (FIG. 4B). It is assumed that the surrounding plane is grounded, i.e. RF spots or lines are alternated by ground planes or strips. The field structure is formed by RF and DC dipoles formed by alternating electrodes. The device is configured to create an array of manipulating cells for ion trapping, conveying, focusing and separating by mass. The method is well compatible with PCB technologies, micromachining, and the small geometrical scale of ion manipulating devices. Unfortunately, opposing RF and DC dipoles substantially limit the mass range of trapped ions.
Summarizing, RF devices are widely used in mass spectrometry for mass analysis and for ion guidance and trapping. A majority of devices have a shape of a 3-D trap or multipole rods. Recently suggested devices employ planar RF surfaces. All the devices are believed to be formed of alternating electrodes aligned on a surface (planar or cylindrical) to form a chain of dipoles. This requires building a structure of alternating electrodes, which complicates fabrication of RF devices and becomes an obstacle to miniaturization and fabrication of massive arrays.